The general objective of this project is to study in detail an in vitro phenomenon which has been termed the isogeneic lymphocyte interaction (ILI). This reaction is characterized by proliferation of neonatal murine thymocytes which have been incubated in mixed culture with isogeneic spleen cells from adult mice. It has been proposed that detailed study of the ILI will provide insight into the role of the thymus in the ontogenesis of the mammalian immune system.